Cardiac rhythm management devices, including implantable devices, are well known in the art. Such devices may include, for example, implantable cardiac pacemakers, cardioverters or defibrillators. The devices are generally implanted in an upper portion of the chest, in either the left or right side depending on the type of the device, beneath the skin of a patient within what is known as a subcutaneous pocket. The implantable devices generally function in association with one or more electrode-carrying leads which are implanted within the heart. The electrodes are typically positioned within the right side of the heart, either the right ventricle or right atrium, or both, for making electrical contact with their designated heart chamber. Conductors within the leads couple the electrodes to the device to enable the device to deliver the desired stimulation therapy.
Traditionally, therapy delivery has been limited to the right side of the heart. The reason for this is that implanted electrodes can cause blood clot formation in some patients. If a blood clot were released from the left-side of the heart, as from the left ventricle, it could pass directly to the brain resulting in a paralyzing or fatal stroke. However, a blood clot released from the right side of the heart, as from the right ventricle, would pass into the lungs where the filtering action of the lungs would prevent a fatal or debilitating embolism in the brain.
Recently, new lead structures and methods have been proposed and even practiced for delivering cardiac rhythm management therapy from or to the left-side of the heart. These lead structures and methods avoid electrode placement within the left atrium and left ventricle of the heart by lead implantation within the coronary sinus and/or the great vein of the heart which communicates with the coronary sinus and extends down towards the apex of the heart. As is well known, the coronary sinus passes closely adjacent the left atrium and extends into the great vein adjacent the left ventricular free wall. The great vein then continues adjacent the left ventricle towards the apex of the heart.
It has been observed that electrodes placed in the coronary sinus and great vein may be used for left atrial pacing, left ventricular pacing, and even cardioversion and defibrillation. This work is being done to address the needs of a patient population with left ventricular dysfunction and congestive heart failure. This patient class has been targeted to receive pacing leads intended for left ventricular pacing, either alone or in conjunction with right ventricular pacing. When delivering such therapy to these patients, it would be desirable to provide device-based measurements of left ventricular function for both monitoring and therapy delivery.
It is known in the art that device-based impedance measurements offer one method for assessing patient condition. It is also well known, however, that bio-impedance measurements can be confounded by signals not directly related to the desired physiology to be measured. For example, a measurement of impedance from a unipolar tip electrode in the right ventricular apex will contain signal components related to respiration, and right ventricular, left ventricular, and aortic hemodynamics. Filtering of the signal can help to isolate the various desired signals, but the filtering required to accurately isolate the desired signals are often not feasible in an implantable cardiac rhythm management device.
It is also known that localization of the desired signals is improved by making proper choice of electrode configurations between which impedance measurements are made. For example, a transchamber impedance technique is known wherein impedance measurements are made between electrodes in the right atrium and right ventricle to assist in isolating the right ventricular hemodynamics.
The advent of cardiac leads for delivering therapy to the left-side of the heart which are often placed in the coronary sinus and great cardiac vein require new techniques for measurement of functional parameters of, or associated with, a heart.